Brother HL-4000CN, Service Manual

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7.8 Fuser Control
7.8.1 Fuser temperature control
As for the fuser temperature control, the target temperature is set, then the heat roll
surface temperature is controlled so as to be the target temperature by turning on/off the
heater lamp.
The heat roll surface temperature is detected by the temp sensor. The temp sensor
detects the temperature and the resistance value for disconnection sensing alternately at
40ms cycles. That is, the temperature is detected at 80ms cycles, and if the detection
result is higher than target value, the heater lamp is turned off, or if lower, the heater
lamp is turned on.
For the target temperature, different temperature is set respectively at the time of
standby, printing, and process control. Also, target temperature is changed according to
the inside temperature detected with the sensor HUM TEMP, print count, print mode, and
input power supply voltage.
7.8.2 Cool down
As the printing continues, the temperature distribution of the heat roll becomes uneven
between paper path and non-paper path. To make the heat roll temperature as even as
possible, the waiting time is provided, during which the heater lamp is not turned on. This
is called the cool down.
The cool down is executed for the duration of certain time according to the number of
sheets continuously printed, when the set printing with certain type and size of paper
completed.
Also, when the paper type or size was changed, the cool down is inserted even in the
midway of several hundred sheets of printing.
As a simple cool down, the PPM down is provided. This function idles the fuser without
feeding the sheets during the printing to improve uneven temperature distribution of the
heat roll.
In printing certain type/size of paper continuously, the page interval is increased from the
specified print counts and the prints per minute are reduced, and thus it is called PPM
down.
The PPM execution timing is determined depending on the paper size and type.